Historically, the measurement of rotor thrust on gas turbine engines has been accomplished by the application of electrical resistance strain gages to an engine bearing housing or bearing race. The electrical output of the strain gage system is proportional to the bearing housing deflection imposed by the rotor thrust loading. Unfortunately, this electrical resistance strain gage technology of the existing art encounters several problems and limitations.
One problem encountered in the prior art is that the strain gage indicated output is dependent upon its temperature environment at any thrust load, thus inducing errors into the measurement. In addition, the strain gages are subject to mechanical fatigue failure and, thus, loss of signal. A third problem is that the strain gages are subject to electrical magnetic interference or other induced electrical noise, thus inducing errors into the thrust load measurement. Also, the strain gages are subject to hookup wiring configuration errors, resulting in a reversal of the indicated rotor thrust direction, meaning that the forward and aft thrust directions can be indicated incorrectly, unknowingly by the observer. A fifth problem encountered in the prior art is that the strain gages are not an absolute measurement, as they require an electrical tare balance and other thermal compensations. Furthermore, the required length of the strain gage extension leads contain a resistive load, producing a voltage drop from the strain gage sensor to the remote data readout system, and thereby producing an error in the thrust measurement. Yet another problem is that the temperature of the strain gage extension leads are subjected to various temperature gradients internally in the engine. This temperature change in the extension leads produces a change in the extension lead resistance, in turn producing an error in the thrust measurement. Finally, the electrical resistance strain gage possesses a calibration constant known as the gage factor. This constant is defined as the unit change of electrical resistance of the strain gage for a given strain input. This gage factor also varies as a function of temperature and can produce an error in the indicated thrust measurement.
It is seen, therefore, that it would be desirable to have an improved rotor thrust measurement apparatus and method which overcomes the problems encountered by the strain gages historically used to measure rotor thrust.